Redirect to login page if refresh token has expired

[eva-mueller-coremedia]

The user should be redirected when the refresh token has expired.

Fixes #457

Changes

• Check if variable is set to avoid NullPointerException
• Use request/response variables instead of Stapler requests

Testing done

• This change has been successfully tested with a AWS Cognito connection.

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 71.73%. Comparing base (2a120e0) to head (84359ac).
Report is 19 commits behind head on master.

Files with missing lines	Patch %	Lines
...va/org/jenkinsci/plugins/oic/OicSecurityRealm.java	50.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #475      +/-   ##
============================================
+ Coverage     71.65%   71.73%   +0.08%     
- Complexity      221      222       +1     
============================================
  Files            17       17              
  Lines          1030     1033       +3     
  Branches        146      148       +2     
============================================
+ Hits            738      741       +3     
+ Misses          203      201       -2     
- Partials         89       91       +2     

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[michael-doubez]

LGTM